Warming Fluctuations Strengthen the Photo-Phagotrophic Coupling in Mixoplanktonic Protists

Abstract

Mixoplankton, a major trophic group in aquatic ecosystems, are being affected by global warming. However, most studies on temperature effects use constant mean conditions, overlooking how short-term thermal fluctuations could deviate from climate projections and impact this group. We experimentally quantified how increasing amplitudes of warming fluctuation (±1, 3, and 5 °C) alter carbon-specific electron transport (ETRc ), net photosynthesis (Pc ), respiration (Rc ), phagotrophy (Phc ), carbon use efficiency (CUE), and growth (µ) in four protist species (three mixoplanktonic and one strict phototroph). We observed a consistent positive link between photosynthetic efficiency (Pc :ETRc ratio) and Phc , and a shift towards a strengthening of the phagotrophy (Pc :ETRc / Phc ratio) with greater thermal fluctuation. A potential explanation is a selective behavior aimed to increase phagotrophy to obtain inorganic nutrients through ingested prey internal re-cycling rather than relying on the environment, to support an enhanced photosynthetic efficiency and growth. An enhanced, coupled photo-phagotrophy activity could boost mixoplankton competitiveness compared to phytoplankton. Our findings underscore the need to incorporate trophic flexibility and its interaction with environmental variability into trait-based models to better predict community dynamics, biogeochemical cycling, and food web structure in aquatic ecosystems.